Tuner testing aero add-ons (they look like prototypes to me, as no-one would have such tacky struts to hold the rear wing in place).

I've been wondering about this for days now, ever since I saw the first link to the pics here in a german M-forum.

Did nobody notice? The rear wing is an adjustable wing, usually found on the 320si (WTCC). Adjustable wings are not street legal in Germany, so there is no point in running them anywhere unless you want to be able to change the aerodynamic balance during a test. Whatever the M GmbH is testing on that car, it has to do with aerodynamics and more on the front end, than on the rear. I would assume, that it is just a future aero kit for the GT4 (which would have to come as an evolution-homologation and has to be purchaseable somewhere) that will create downforce rather than lift.